The influence of population density on salt preference was investigated in Sprague-Dawley rats. In experiment I, after 12 days of housing at one rat per cage, 12 female rats were divided randomly into two groups, i.e. a control group remaining at one rat per cage, and a crowded group housed at 3 rats per cage. The rats were observed for 21 days. Distilled water, and 0.5%, 1.0%, 1.5% saline solution were supplied ad libitum with salt-free feed. The salt intake of rats in the control groups did not change during the whole period. On the other hand, the salt intake of rats in the crowded group decreased significantly from 0.29±0.03 to 0.18±0.01g/rat/day (mean±SE, p <0.01) when switched from one rat per cage to 3 rats per cage. A similar study (Exp. II) was done using 20 male rats. The salt intake of rats in the crowded group also decreased from 0.41±0.02 to 0.31±0.02g/rat/day ( p < 0.005) when switched from one rat per cage to 3 rats per cage. These results suggest that an environment in which one rat is housed per cage may be more stressful than one in which 3 rats are housed per cage, and may cause an increased preference for salt in these solitary rats.